1. Field of the Invention
The present invention pertains to a control device for a power transmission device for a vehicle, and relates to a technique for using a plurality of types of fuel to drive an internal combustion engine.
2. Description of the Related Art
Conventionally, there has been known a power transmission device for a vehicle including a differential part that has a differential mechanism coupled between an internal combustion engine and driving wheels and a first motor coupled to the differential mechanism and that controls the differential state of the differential mechanism by controlling the operating state of the first motor, a speed shift transmission coupled between the differential part and the driving wheels, a second motor coupled to the input side of the speed shift transmission, and a differential restriction device capable of selectively switching the differential mechanism between a non-differential state in which its differential action is disabled and a differential enabled state in which its differential action is enabled. Such a power transmission device for a vehicle may be suitably used in hybrid vehicles, and is disclosed in Japanese Patent Application Publication No. 2005-273900 (JP-A-2005-273900), for example. A control device for the power transmission device for a vehicle according to JP-A-2005-273900 switches the differential mechanism between the non-differential state and the differential enabled state using the differential restriction device on the basis of predetermined conditions using the vehicle speed etc. as parameters.
In recent years, a plurality of types of fuel are occasionally used to drive an internal combustion engine. When different types of fuel are used to drive the internal combustion engine, the output characteristics of the internal combustion engine are also different. For example, in the case where the internal combustion engine is driven on an ethanol-containing fuel obtained by mixing ethanol into a gasoline fuel, the anti-knocking performance of the internal combustion engine is improved and the ignition timing of the engine is thus advanced, which tends to increase the output torque of the engine, compared to the case where the internal combustion engine is driven on the gasoline fuel. Thus, in order to maintain the operating efficiency of the entire vehicle at a high level, it is necessary that the predetermined conditions for switching the differential mechanism between the non-differential state and the differential enabled state should be modified in accordance with changes in output characteristics of the internal combustion engine due to changes in type of fuel. However, the control device according to JP-A-2005-273900 particularly does not modify the predetermined conditions and hence does not reduce the fuel consumption rate in correspondence with a plurality of types of fuel to be supplied. Such an issue is not known in the art.